Original equipment manufacturers (OEMs) providing centrifugal compressors for the process market industry, e.g., oil and gas, petrochemical, gas transmission applications, and the like, have seen an increasing demand for stages of the centrifugal compressors operating at higher flow coefficients and higher machine or inlet relative Mach numbers. Such demands are typically driven by a desire to reduce the footprint of the compressor or to compress larger amounts of gas within a smaller casing. As a direct result, many process centrifugal compressors now operate at machine Mach numbers, U2/A0s, in excess of 1.2 and shroud relative Mach numbers of 0.95 and higher.
In designing such smaller or higher capacity centrifugal compressors, focus is generally directed to the impeller design, and in addition, the design of the stationary components, such as the diffuser. In operation, common issues resulting from improper diffuser design are instabilities known as surge and rotating stall. Typically, rotating stall occurs because the design of the diffuser, in many cases a vaneless diffuser, is unable to accommodate all flow without some of the flow experiencing separation in the diffuser passageway. Rotating stall results in the creation of low frequency pulsations at fundamental frequencies generally less than the rotating frequency of the impeller. Such lower frequency pulsations or vibrations may propagate downstream through the gas passageways and potentially result in performance degradation in the centrifugal compressor, the control system of the centrifugal compressor, and/or associated components. Rotating stall is also recognized as a precursor to surge. Surge is a far more violent event that can cause premature failure of compressor components.
Surge/stall detection and avoidance systems have been proposed to reduce or eliminate the occurrence of rotating stall and/or surge in centrifugal compressors. In particular, some of the aforementioned systems rely on external instrumentation to measure inlet and outlet gas flow properties; however, such external instrumentation may be subjected to undesirable external conditions. Other systems rely on the measurement of acoustic energy in the gas stream to detect a surge or rotating stall. However, such systems may be subject to the vibrations provided by the rotating stall, thereby reducing the longevity of the system.
What is needed, then, is an efficient and reliable system and method of detecting an impending rotating stall and/or surge before the actual occurrence of the rotating stall and/or surge.